Various types of polling techniques have been developed in the communication art. A controller can be connected over a single communication path (such as a pair of conductors) to a plurality of transponders, so that when any transponder goes into alarm the controller "knows" that some alarm condition has occurred, but does not know where the alarm event has taken place or which transponder is replying. An improvement over that system is an arrangement in which each transponder or passive alarm unit transmits a combination of an alarm signal with an identification of its location (its "address"). This can be accomplished without any polling signal from the controller, but it has a substantial drawback in that the controller does not recognize when one of the transponders becomes inoperative before it is required to transmit an alarm signal. For this reason improved systems have been developed in which the individual transponder units all have their individual respective addresses, which differ from each other, and the various alarm or transponder units can reply to the controller when addressed. In this way the controller knows when a transponder has a defect or trouble condition when it does not reply to a normal inquiry, even when no alarm or danger condition is present adjacent the addressed transponder.
In such polling arrangements, the transponders are frequently addressed in sequence. For example if there are 60 transponders connected over a single communication path to a controller, the first transponder is addressed and given time to reply, the second is then addressed, and so forth through the entire 60 units. In this way the controller is continually checking on the operability as well as the alarm status of each of the units. Another way to address the transponders is by generating the addresses in a random, rather than a sequential, manner.
A complete teaching of effective sequential polling arrangements is set out in U.S. Pat. Nos. 4,394,655, issued July 19, 1983; 4,470,047, issued Sept. 4, 1984; and 4,507,652, issued Mar. 26, 1985. All these patents are entitled "Bidirectional, Interactive Fire Detection System", and all are assigned to the assignee of this invention. In addition a technique for utilizing such polling systems to expand the amount and/or significance of the data transferred is described and claimed in application Ser. No. 716,799, filed Mar. 27, 1985, which issued Apr. 14, 1987 as U.S. Pat. No. 4,658,249 and is assigned to the assignee of this invention. The disclosures of these teachings, including the bidirectional and interactive features, are incorporated herein by reference.
With either polling technique (sequential or random) a finite time period, even though only a few seconds, is required to complete one polling sequence. This is a severe penalty when one or more of the alarm units are connected to transmit high-priority alarm information, such as "holdup in progress". For example if the 17th transponder out of 60 is connected to transmit the "holdup" signal and the 18th transponder is being polled when the holdup signal is initiated at transponder number 17, then the polling sequence must be completed and restarted, going back to number 17 before the holdup condition is recognized. If an actual bank robbery were in progress, this is an unacceptable delay.
It is thus a primary consideration of the present invention to provide a communication system of either the sequential or random polling type, in which the normal polling sequence is overridden when a high priority message is initiated at a given station.
A corollary consideration of the present invention is to provide such an improved system in which one or more subsets or small groups of transponders, within a larger group of transponders, can be connected for the high-priority interrupt operation, without necessitating such operation of all the transponders in the system.
Another important consideration of the present invention is to substantially eliminate the delay otherwise attendant upon the high-priority interrupt system, so that the alarm equipment such as surveillance cameras can be energized and in operation within a second after the high-priority alarm is initiated.